Conventional analog weather radar units carried in airships have for many years featured an iso-contour mode. In this mode, clouds of the type which are likely to contain a strong windshear (storm clouds) are displayed as a light fringe with a dark hole in the center. Such a display is accomplished by looking for radar video return signals above a certain preselected value (termed the iso-contour level) and, during the time so received, clamping the video signal passed to the display unit to a base line. The comparator device utilized to detect when the iso-contour level has been reached, in order to achieve noise immunity, creates a hysteresis which, undesirably, has the effect of allowing the leading edge of a cloud video signal to rise above the iso-contour level before cutoff of the video signal to the display occurs, while not turning the video signal back on until the signal has often dropped well below the iso-contour level.
With conventional direct storage tube display devices, such hysteresis provided little or no perceived display deficiency. The lack of deficiency is due to the phosphor persistance on the tube and resulting frame-to-frame integration possible with a continuum of intensity levels. The only indication of any deficiency, and then only to a skilled operator, is a slight drop in intensity level at the trailing edge of the cloud. With the newer digital type indicators and resultant quantization of video signals, an erroneous display in the form of an open ended cloud presentation can occur when the radar is set to relatively greater maximum ranges. As will be brought out in greater detail in the detailed description section of the instant application, the quantization process at greater range intervals may result in the trailing edge portion of an iso-contour not being displayed.